Ball valves typically comprise of a generally spherical valve element, with a bore extending therethrough. The valve element is rotatably mounted in a housing which has aligned openings on either side, one for entry of fluid into the housing and one for exit of fluid from the housing. The valve element may be rotated such that the bore may either be aligned in the same direction as the openings or perpendicular to the openings. In the perpendicular orientation flow through the valve is prevented, while in the parallel direction flow of fluid through the valve is allowed. Each of the openings of the housing is provided with a sealing seat assembly for sealing between the opening and the ball valve element.
The seat assemblies are classified as either hard or soft, based upon whether they are integral with the valve body (hard) or made of a different, softer material (soft). Traditional top entry ball valves employ various complicated ball and seat assembly designs. Examples of typical assemblies include wedge designs, caged ball designs, and trunnion designs.
A wedge design requires the valve seats to be inserted in the valve body at an angle to the axis of the aligned openings. The bore of the ball is machined with a taper in order to interact with the degree of inclination of the seats. A spring is provided on the valve stem to force the ball towards the valve seats. An adjustment device is provided in order to keep the assembly in place and to allow adjustment of the assembly. This design requires skilled technicians for assembly and disassembly due to its complex nature.
Caged ball designs involve the complete assembly of ball and both seats being put in to a cage. This cage is attached to the valve cover and inserted into the valve body. This design requires a complex valve body shape, which can only be manufactured by casting the valve. If a forged valve body is used, special complicated machinery is required to manufacture suitable shapes. This therefore means that the cost of manufacturing such valve is prohibitively large.
Trunnion seat assemblies mechanically anchor the ball at its top and bottom. These assemblies require extra components to work, and add two leakage paths between the seat assembly and the valve body. Additionally, the designs are typically difficult to assemble, disassemble and service.
These traditional designs and assembly methods suffer from a number of common problems. These include inadvertently generating multiple leakage paths due to the relatively complex construction. In addition, special tools are often required which are solely supplied by the valve manufacturer. Such tools typically only work for a single size of valve and as such may vastly increase the maintenance and construction costs of such valves. These tools require trained and skilled operators.